1. Field of the Invention
This invention is concerned with an improved process for converting synthesis gas, i.e. mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures. This invention is particularly concerned with a process for converting synthesis gas to hydrocarbon mixtures rich in aromatic hydrocarbons. In another aspect, this invention is concerned with providing novel catalysts for the conversion of synthesis gas to hydrocarbon mixtures rich in aromatic hydrocarbons.
2. Prior Art
Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. Although various processes may be employed for the gasification, those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433, (1966), Interscience Publishers, New York, New York, the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, liquid or gaseous fuel are not considered to be per se inventive here.
It would be very desirable to be able to effectively convert synthesis gas, and thereby coal and natural gas, to highly valued hydrocarbons such as motor gasoline with high octane number, petrochemical feedstocks, liquefiable petroleum fuel gas, and aromatic hydrocarbons. It is well known that synthesis gas will undergo conversion to form reduction products of carbon monoxide, such as hydrocarbons, and/or oxygen containing compounds such as methanol at from about 300.degree. F. to about 850.degree. F. under from about one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of liquid hydrocarbons, a portion of which have been used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals, oxides or other compounds of iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium. Methanol synthesis processes, for example, use catalysts composed of mixtures of two or more oxides and in particular use ZnO base and CuO base mixed catalysts. A review of catalytic processes for the synthesis of methanol from mixtures containing CO and H.sub.2 is given in Emmett, P.H., Catalysis III, N. Y., Reinhold, 1955. Chapter 8, pages 349-411, by G. Natta, Synthesis of Methanol, the text of which is incorporated herein by reference.
The wide range of catalysts and catalyst modifications disclosed in the art and an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen provide considerable flexibility toward obtaining selected boiling-range products. Nonetheless, in spite of this flexibility, it has not proved possible to make such selections so as to produce liquid hydrocarbons in the gasoline boiling range which contain highly branched paraffins and substantial quantities of aromatic hydrocarbons, both of which are desired for high quality gasoline, or to selectively produce aromatic hydrocarbons particularly rich in the benzene to xylenes range. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions", Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 4, pages 446-488 and and Volume 13, pages 370-398. Interscience Publishers, New York, N. Y., the text of which is incorporated herein by reference.
Recently it has been discovered that synthesis gas may be converted to oxygenated organic compounds and these then converted to higher hydrocarbons, particularly high octane gasoline, by catalytic contact of the synthesis gas with a carbon monoxide reduction catalyst followed by contacting the conversion products so produced with a special type of zeolite catalyst in a separate reaction zone. This two-stage conversion is described in copending U.S. patent application, Ser. No. 387,220, filed on Aug. 9, 1973.
Still more recently, it has been discovered that synthesis gas may be converted to hydrocarbon mixtures useful in the manufacture of heating oils, gasoline, aromatic hydrocarbons, and chemical intermediates by catalytic contact with an intimate mixture of: (1) carbon monoxide hydrogen reduction catalyst comprising a methanol synthesis catalyst and (2) a special type of zeolite catalyst comprising an acidic crystalline aluminosilicate having a silica:alumina ratio greater than 12 and a pore dimension greater than about 5 Angstroms. This one-stage conversion is described in copending U.S. Patent application Ser. No. 730,871, filed on Oct. 8, 1976.
It is an object of the present invention to provide an improved process for converting fossil fuels to a hydrocarbon mixture that contains large quantities of highly desirable constituents. It is a further object of this invention to provide a more efficient method for converting a mixture of gaseous carbon oxides and hydrogen to a mixture of hydrocarbons. It is a further object of this invention to provide an improved method for converting synthesis gas to a hydrocarbon mixture rich in aromatic hydrocarbons. It is a further object of this invention to provide novel catalysts for the conversion of synthesis gas to a hydrocarbon mixture rich in aromatic hydrocarbons.